With the crop-tool active in ACR, right click on your image and you can choose from any of several predefined ratios, or define your own custom ratio. (I believe there's even a 'normal' option in which no ratio is enforced at all).

When you select the crop tool, and drag it out from the upper left to the lower right you'll have the crop set to the exact same size as your capture...then if you hold the shift key and drag a corner, you can crop while maintaining the same ratio...if you hold the shift and option (alt) you can decrease or increase the crop from the center while still maintaining the ratio...

The crop tool having 'custom' sizes - pixel' inches, etc. was one of the great upgrades in ACR. Absolutely love it.One of the best options is to select the whole shoot, then custom crop them all at once.